Oil sealed, mechanical rotary vacuum vane pumps have been extensively utilized in the past, both as primary pumps for vacuum loads of 10.sup.-4 -10.sup.-1 mm. of mercury, or as fore pumps in combination with diffusion pumps for vacuum loads of greater than 10.sup.-4 mm. of mercury. Such pumps usually include several stages, referred to as high and low vacuum stages. Each stage includes a plurality of vanes that cyclically rotate about the axis of a rotor eccentrically mounted in a cylindrical bore of a stator. As the vanes turn, they pump gas between inlet and outlet ports that are physically close to each other in the stator, but which are as far as possible from each other in the pumped gas flow path. To this end, the rotor and stator are designed so there is a very narrow gap between them in a region bridging the inlet and outlet ports. An oil seal is formed in this region by a dam for the pumped gas. The dam is formed by oil supplied to the pump for sealing and lubrication purposes. In a two stage pump, the rotors of the high and low vacuum stages are drivingly connected by a shaft that extends through an interstage structure between the high and low vacuum stages.
As disclosed in the textbook High Vacuum Pumping Equipment by B. D. Power, 1966, Reinhold Publishing Corporation, New York, page 18, it is known to intermittently feed oil to a vacuum pump by way of metering flats on a shaft for the rotor of one of the stages. The flats are diametrically opposite from each other and aligned with radially extending passages. One passage leads from a source of oil, on one side of the shaft, while the other passage leads to one of the stages on the other side of the shaft. This prior art device has several advantages because the amount of oil supplied to the stages is a direct function of the sealing requirements of the stage, due to the coupling of a metering mechanism directly to the rotor.
However, the radial disposition of both passages for feeding oil to the shaft and for feeding oil from the shaft to the vacuum stages has a substantial disadvantage. In particular, oil droplets supplied by a metering flat to the conduit leading to the vacuum stage encounter a relatively high flow impedance. The high flow impedance occurs because the droplets have a tendency to impinge on a wall of the conduit leading from the flat. Because the oil droplets impinge on the wall, a dam has a tendency to be formed immediately downstream of the flat, frequently preventing the flow of oil to the vacuum stage until the conduit has been filled.